The present invention relates to means and methods for preventing the exfoliation of highly graphitic carbonaceous electrode materials used in electrochemical cells, which exfoliation is associated with the decomposition of the cell electrolyte solvent. In particular, the present invention relates to electrochemical cells having electrodes of highly graphitic carbonaceous materials and electrolyte solutions to which sequestering agents have been added to prevent co-intercalation of the electrolyte solvent into the graphite electrode during initial cell discharge, which co-intercalation causes exfoliation of the graphite electrode.
Electrochemical cells useful as electrical storage batteries usually incorporate a metal-containing anode and a cathode including an active material which can take up ions of the metal. An electrolyte incorporating ions of the metal is disposed in contact with the anode and the cathode. During discharge of the cell, metal ions leave the anode, enter the electrolyte and are taken up in the active material of the cathode, resulting in the release of electrical energy. Provided that the reaction between the metal ions and the cathode-active material is reversible, the process can be reversed by applying electrical energy to the cell. If such a reversible cathode-active material is provided in a cell having the appropriate physical configuration and an appropriate electrolyte, the cell can be recharged and reused. Rechargeable cells are commonly referred to in the battery art as "secondary" cells.
It has long been known that useful secondary cells can be made using a light alkaline metal such as sodium, potassium, and particularly, lithium, as the source of the metal ions exchanged between the anode and cathode through the electrolyte. These metals are particularly useful in combination with a cathode-active material that is a sulfide or oxide of a transition metal, i.e., a metal capable of assuming plural different valence states. In the past, these alkaline metals such as lithium have been used in electrochemical cells in their pure metal state, as the cell counterelectrode in combination with the transition metal cathode-active material. See, for example, Dampier, J. Electrochem. Soc., 121(5), 656-660 (1974). It is common knowledge that water reacts violently with alkaline metals such as sodium, potassium and lithium in their pure metal state. Not only must water be excluded from any component of the cell having an alkaline metal counterelectrode, extreme care must be taken during cell assembly to avoid exposure of the counterelectrode metal material to ambient moisture and other sources of water.
Secondary lithium cell researchers have sought to develop a rechargeable lithium cell containing no metallic lithium. Cells have been developed using instead of a lithium metal anode, a lithium intercalation host that operates near the potential of lithium, such as the carbonaceous materials and cells incorporating same disclosed in presently co-pending U.S. patent application Ser. No. 350,396 by Fong et al., filed May 11, 1989, the disclosure of which application is hereby incorporated herein by reference thereto.
Replacing lithium metal counterelectrodes with lithium intercalation host counterelectrodes removes the restrictions lithium metal electrodes place upon cell design and choice of electrolytes and also the adverse effect lithium metal places upon cycling performance and safety in the finished cell. Highly graphitic carbonaceous materials are ideal lithium intercalation hosts. Highly graphitic carbonaceous materials such as graphite are inexpensive, non-toxic and are capable of incorporation into electrochemical cells having relatively high specific capacities. As noted in the above-cited U.S. patent application Ser. No. 350,396, the drawback to use of such materials is that upon the initial charging of the cell, when lithium is intercalated into the host, an irreversible reaction occurs in which lithium and the cell electrolyte solvent are consumed, resulting in an initial capacity loss for the cell and a reduction of the cell's overall performance. The above patent application correlates this reaction to the tendency of highly graphitic carbonaceous intercalation hosts to exfoliate when initially intercalated with lithium metal. The cited application defines exfoliation as the change in an intercalation host material resulting in an increase in its surface area subsequent to intercalation with lithium metal as compared to the surface area prior to intercalation. It is disclosed in the cited application that highly graphitic carbonaceous materials have an organized layered structure, which layers are easily separated or exfoliated. It is disclosed that the amount of electrolyte consumed upon initial intercalation is substantially proportional to the surface area of the carbonaceous intercalation host, so that the exfoliation of highly graphitic carbonaceous intercalation hosts upon initial charging results in the consumption of greater electrolyte solvent and loss of cell capacity and performance properties than occurs with carbonaceous or other intercalation hosts that do not suffer from exfoliation.
U.S. patent application Ser. No. 350,396 offers two solutions to the problem of exfoliation of highly graphitic carbonaceous intercalation hosts to prevent the excessive consumption of cell electrolyte solvent and consequent loss of cell capacity and performance properties. The first is to form a dual phase carbonaceous intercalation host having a mean degree of graphitization of at least about 0.40, with one phase having a degree of graphitization greater than 0.40 and the other phase having a degree of graphitization less than about 0.40. The other approach maintains the carbonaceous intercalation host at a temperature greater than about 50.degree. C. during the initial intercalation of the host with lithium. Both approaches provide a carbonaceous intercalation host resistant to exfoliation during the initial lithium intercalation. However, a solution to the exfoliation problem that does not require the replacement of single-phase highly graphitic intercalation hosts or the heat treatment of same would be highly desirable.
Co-intercalation of electrolyte solvent with lithium into TiS.sub.2 and ZrS.sub.2 host electrodes is disclosed by McKinnon, J. Electrochem. Soc., 132(2), 364 (1985). Morita et al., J. Electro. Soc., 134(9), 2107 (1987) disclose that the addition of crown ether sequestering agents to electrolyte solutions limits solvent co-intercalation into TiS.sub.2 electrode hosts. U.S. Pat. No. 4,132,837 to Soffer discloses that the addition of sequestering agents such as crown ethers to electrolyte solutions reduces the tendency of the electrolyte solvent to degrade in the presence of lithium metal anodes. None of these references, however, provide any guidance as to how to solve the exfoliation problem.